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Chapter 7: Problem 98

Why does rubbing hand sanitizer into your hands cause it to disappear faster?

Short Answer

Expert verified
Rubbing hand sanitizer increases evaporation by adding heat and surface area through friction.

Step by step solution

01

Understand the Composition of Hand Sanitizer

Hand sanitizer is primarily composed of alcohol, usually ethyl or isopropyl alcohol, which is volatile, meaning it easily vaporizes at room temperature. It typically contains other ingredients like water and glycerol.
02

Identify the Role of Rubbing

Rubbing your hands together increases the surface area for evaporation and adds energy through friction. This energy helps break the intermolecular bonds, allowing alcohol molecules to escape more quickly into the air.
03

Consider Temperature Increase

The friction from rubbing your hands also generates heat, which raises the temperature of the sanitizer slightly. This increase in temperature provides additional energy, encouraging faster evaporation of the alcohol.
04

Compare to Standing Still

If you apply hand sanitizer and let it sit without rubbing, the evaporation process relies solely on atmospheric conditions. Rubbing manually accelerates the process by combining both physical and thermal energy.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alcohol Volatility
Volatility is a key property of alcohol that explains why hand sanitizer disappears so quickly. Alcohol, such as ethyl or isopropyl alcohol, has low boiling points, which means it transitions from a liquid state to a gas at relatively low temperatures. When alcohol is used in sanitizers, its volatile nature makes it useful for quick drying.
  • Volatile substances like alcohol are quick to evaporate.
  • This rapid evaporation is ideal for sanitizers as it leaves no residue.
  • The molecules of alcohol escape into the air as vapor.
The easier it is for a substance to evaporate, the faster it can disinfect and then disappear from your skin. This is especially helpful in sanitization, where quick drying means no sticky after-feel.
Friction and Heat
Rubbing your hands together creates friction, a force that not only warms up your hands but also plays a role in the evaporation of alcohol. Friction occurs when two surfaces move against each other, generating heat as a byproduct. This heat provides additional energy to the molecules of the hand sanitizer.
  • Rubbing generates heat through friction.
  • This heat increases the temperature of the sanitizer slightly.
  • The added energy helps break molecular bonds more easily.
The increase in temperature accelerates the evaporation rate of the alcohol. Without friction, the alcohol might evaporate much slower, depending solely on room temperature to provide the necessary energy.
Intermolecular Bonds
Intermolecular bonds are the forces that keep molecules together in a substance. In liquids, these bonds can be weaker than in solids, especially for volatile substances like alcohol. When you rub sanitizer onto your hands, the friction and resulting heat provide the energy needed to overcome these bonds, allowing the alcohol to evaporate more quickly.
  • Alcohol has weaker intermolecular bonds compared to water.
  • Once these bonds are broken, alcohol molecules can easily escape.
  • Increased energy from friction breaks these bonds more efficiently.
Breaking these bonds is crucial for the fast evaporation rate of alcohol. This efficient breaking of bonds is why hand sanitizer dries quickly and evaporates without a trace, leaving behind clean and sanitized hands.

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